These rocks likely began as sediments at the bottom of the Iapetus Ocean, were buried and metamorphosed (changed by heat and pressure) by the closing of this ocean basin (during formation of Pangaea), and were then exposed by uplift and erosion. Quartz veins (both pre- and syn- metamorphism) as well as other minerals (staurolites, especially) can be used to determine the depth of the journey these rocks took.